Investments - Aging of Unrealized Losses on Investments in Fixed Maturities and Equity Securities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Unsettled purchases or sales of Investments in fixed maturities	$ 409,600,000	$ 329,800,000
Other receivables	187,000,000	193,100,000
Fair Value
Less Than 12 Months	423,600,000	1,199,100,000
12 Months or Longer	580,700,000	119,600,000
Total	1,004,300,000	1,318,700,000
Unrealized Losses
Less Than 12 Months	(9,700,000)	(76,000,000)
12 Months or Longer	(15,000,000)	(9,900,000)
Total	(24,700,000)	(85,900,000)
Unrealized losses related to securities for which the Company has recognized credit losses in earnings	0	0
Securities that have had recognized credit losses in earnings, aggregate losses		(300,000)
US Government and Government Agencies and Authorities [Member]
Fair Value
Less Than 12 Months	24,900,000	95,800,000
12 Months or Longer	55,500,000	4,400,000
Total	80,400,000	100,200,000
Unrealized Losses
Less Than 12 Months	(700,000)	(10,900,000)
12 Months or Longer	(1,300,000)	(800,000)
Total	(2,000,000)	(11,700,000)
States and Political Subdivisions [Member]
Fair Value
Less Than 12 Months	1,000,000	222,900,000
12 Months or Longer	126,300,000	2,000,000
Total	127,300,000	224,900,000
Unrealized Losses
Less Than 12 Months	0	(20,100,000)
12 Months or Longer	(1,800,000)	(100,000)
Total	(1,800,000)	(20,200,000)
Corporate Bonds and Notes [Member]
Fair Value
Less Than 12 Months	250,400,000	699,800,000
12 Months or Longer	360,500,000	103,200,000
Total	610,900,000	803,000,000
Unrealized Losses
Less Than 12 Months	(5,100,000)	(39,400,000)
12 Months or Longer	(10,000,000)	(8,300,000)
Total	(15,100,000)	(47,700,000)
Collateralized Loan Obligations [Member]
Fair Value
Less Than 12 Months	51,200,000
12 Months or Longer	3,400,000
Total	54,600,000
Unrealized Losses
Less Than 12 Months	(700,000)
12 Months or Longer	(100,000)
Total	(800,000)
Other Mortgage and Asset-backed Securities [Member]
Fair Value
Less Than 12 Months	0	13,900,000
12 Months or Longer	400,000	1,100,000
Total	400,000	15,000,000
Unrealized Losses
Less Than 12 Months	0	(100,000)
12 Months or Longer	0	0
Total	0	(100,000)
Investments in Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Unsettled purchases or sales of Investments in fixed maturities	0	0
Other receivables		2,500,000
Fair Value
Less Than 12 Months	327,500,000	1,032,400,000
12 Months or Longer	546,100,000	110,700,000
Total	873,600,000	1,143,100,000
Unrealized Losses
Less Than 12 Months	(6,500,000)	(70,500,000)
12 Months or Longer	(13,200,000)	(9,200,000)
Total	(19,700,000)	(79,700,000)
Investments in Fixed Maturities [Member] | External Credit Rating, Investment Grade [Member]
Unrealized Losses
Total	(14,100,000)	(74,200,000)
Investments in Fixed Maturities [Member] | External Credit Rating, Non Investment Grade [Member]
Unrealized Losses
Total	(5,600,000)	(5,500,000)
Percentage of unrealized loss position to amortized cost basis of available for sale security average	4.00%	5.00%
Preferred Stock [Member] | Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	7,500,000	22,500,000
12 Months or Longer	12,100,000	2,500,000
Total	19,600,000	25,000,000
Unrealized Losses
Less Than 12 Months	(100,000)	(2,500,000)
12 Months or Longer	(900,000)	0
Total	(1,000,000)	(2,500,000)
Preferred Stock [Member] | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value
Less Than 12 Months	0	4,300,000
12 Months or Longer	500,000	700,000
Total	500,000	5,000,000
Unrealized Losses
Less Than 12 Months	0	(100,000)
12 Months or Longer	0	0
Total	0	(100,000)
Common Stock [Member] | 2000 to 3999 Manufacturing [Member]
Fair Value
Less Than 12 Months	15,100,000	5,000,000
12 Months or Longer	0	200,000
Total	15,100,000	5,200,000
Unrealized Losses
Less Than 12 Months	(1,100,000)	(100,000)
12 Months or Longer	0	0
Total	(1,100,000)	(100,000)
Common Stock [Member] | All Industries Other than Manufacturing [Member]
Fair Value
Less Than 12 Months	4,200,000	14,200,000
12 Months or Longer	1,000,000	500,000
Total	5,200,000	14,700,000
Unrealized Losses
Less Than 12 Months	(400,000)	(900,000)
12 Months or Longer	0	0
Total	(400,000)	(900,000)
Exchange Traded Funds [Member]
Fair Value
Less Than 12 Months	14,900,000	67,600,000
12 Months or Longer	14,400,000	0
Total	29,300,000	67,600,000
Unrealized Losses
Less Than 12 Months	(100,000)	(1,000,000)
12 Months or Longer	(600,000)	0
Total	(700,000)	(1,000,000)
Limited Liability Companies and Limited Partnerships [Member]
Fair Value
Less Than 12 Months	54,400,000	53,100,000
12 Months or Longer	6,600,000	5,000,000
Total	61,000,000	58,100,000
Unrealized Losses
Less Than 12 Months	(1,500,000)	(900,000)
12 Months or Longer	(300,000)	(700,000)
Total	(1,800,000)	(1,600,000)
Equity Securities [Member]
Fair Value
Less Than 12 Months	96,100,000	166,700,000
12 Months or Longer	34,600,000	8,900,000
Total	130,700,000	175,600,000
Unrealized Losses
Less Than 12 Months	(3,200,000)	(5,500,000)
12 Months or Longer	(1,800,000)	(700,000)
Total	$ (5,000,000)	$ (6,200,000)